A known hydraulic drive system for a construction machine such as a hydraulic excavator controls a delivery flow rate of a hydraulic pump (one pump) such that a delivery pressure of the hydraulic pump is higher by a target differential pressure than a maximum load pressure of a plurality of actuators. This control is called load sensing control. The hydraulic drive system that performs the load sensing control includes a plurality of pressure compensating valves, each of the pressure compensating valves maintaining, as disclosed in Patent Document 1, a predetermined differential pressure across each of respective flow control valves. The hydraulic drive system is thereby be able, in a combined operation in which the actuators are driven simultaneously, to supply each of the actuators with a hydraulic fluid at a ratio corresponding to an opening area of each flow control valve regardless of the magnitude of a load pressure on each of the actuators.